JPH0865352A - Digital modulator - Google Patents

Abstract

PURPOSE: To efficiently send a digital signal by interposing a compensating circuit which has characteristics opposite from amplitude characteristics and phase characteristics of a power amplification part between a mapping circuit and a waveform shaping circuit which output an in-phase signal and an orthogonal signal. CONSTITUTION: The compensating circuit 2 which has the characteristics opposite from the phase characteristics and amplitude characteristics of the power amplification part 8 of a satellite is connected before the waveform shaping circuit 3. A signal is inputted to the mapping circuit 1, whose output is inputted to the circuit 2; and the circuit 2 outputs a signal which is converted with the opposite characteristics of the amplifier 8 and widened in band. This signal has its signal waveform shaped by the circuit 3 and its band width limited and is outputted. This signal is sent to the satellite and amplified by the amplification part 8 mounted on the satellite. At certain time, the signal is distorted because of the nonlinearity of the amplification part 8, but the distortion is small since the signal is compensated with the opposite characteristics. Consequently, the signal is 3.7dB improved and a twice amount of information can be sent with the same frequency band width.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital modulator used for transmitting a digital signal through a transmission line having a non-linear amplifier such as satellite communication and satellite broadcasting.

[0002]

2. Description of the Related Art Conventionally, phase modulators and quadrature amplitude modulators (QAM) have been used as modulators used for transmitting digital signals. In particular, in the field of land mobile communication such as mobile phones, frequency utilization efficiency is high. High QAMs are beginning to be used. However, power amplifiers (traveling wave tubes) used in satellite broadcasting and satellite communications are often used in the saturation region by giving priority to power efficiency, and QAM that carries information on the amplitude is greatly affected by non-linearity. I can't use it.

In order to reduce the influence of non-linearity, conventionally, the operating point of the power amplifier is lowered and used, or as shown in the circuit configuration diagram of FIG. I was trying to make it smaller. In FIG. 3, reference numeral 11 is a mapping circuit having a function of inputting a digital signal and outputting an in-phase signal I and a quadrature signal Q corresponding to the digital signal, and a reference numeral 12 is an input between the in-phase signal I and the quadrature signal Q and between codes. A waveform shaping circuit having a function of shaping the waveform of the signal so that interference does not occur, reference numeral 13 is a quadrature modulation circuit having a function of inputting the waveform-shaped in-phase signal I and quadrature signal Q and performing quadrature modulation, Reference numeral 14 is a frequency conversion circuit having a function of converting the frequency of the signal from an intermediate frequency to a high frequency for transmission, reference numeral 15 is a ground station power amplification circuit having a function of amplifying the power of the high frequency signal, and reference numeral 100 is for reducing the influence of non-linearity. It is a conventional compensation circuit having a function of

[0004]

When the operating point of the power amplifier is lowered by the conventional method, the transmission power is also lowered and the power efficiency of the amplifier is deteriorated. The power amplifier mounted on the satellite must effectively use the limited electric power generated by the solar cell of the satellite, and the decrease in efficiency has been a problem. Further, even when the pre-compensation circuit is used, there is a drawback that the bandwidth of the transmission signal is widened because the compensation circuit is behind the waveform shaping circuit. Furthermore, the effect of intersymbol interference due to non-linear amplification could not be eliminated. Therefore, the error rate at the time of transmitting a digital signal becomes large, and it is not possible to use a modulation method with good transmission efficiency.

The present invention is a line that uses a non-linear power amplifier such as satellite broadcasting and satellite communication, and performs non-linear compensation of a signal without changing the operating point of the power amplifier and without widening the bandwidth of a transmission signal. It is an object of the present invention to provide a circuit for performing efficient transmission of digital signals by reducing the influence of intersymbol interference.

[0006]

In order to achieve the above object, the digital modulator of the present invention has an inverse characteristic of an amplitude characteristic and a phase characteristic of a power amplifier (8) used for power amplification of a transmission signal to be received. The compensating circuit (2) having the above is connected between the waveform shaping circuit (3) and the mapping circuit (1) which inputs the digital signal and outputs the in-phase signal and the quadrature signal.

[0007]

When a digital signal is input to the compensating circuit 2 configured as described above, the output of the power amplifier 8 becomes a signal with small non-linear distortion, as will be described below.

The amplitude of the input signal is a, the amplitude characteristic of the power amplifier is A (a), the phase characteristic of the power amplifier is θ (a), the amplitude characteristic of the compensation circuit is C (a), and the phase characteristic of the compensation circuit is φ.
Assuming (a), the characteristics of the power amplifier and the compensation circuit are A
(C (a)) = a and θ (C (a)) = − φ
The relationship (a) is satisfied. FIG. 4 shows a characteristic example of the power amplifier. Further, an example of characteristics of the compensation circuit is shown in FIG.

On the phase plane represented by the symbol arrangement diagram shown in FIG. 6, the position of the signal before being affected by the nonlinearity is 20,
The position after receiving is 21. Without the non-linear compensation, the amplitude would shift to A (a) and the phase would shift by θ due to the non-linearity of the power amplifier. Therefore, a compensating circuit shifts in advance to the point 22 of the amplitude C (a) and the phase difference φ. After passing through the power amplifier, the amplitude is A (C
(A)), the phase difference is θ (C (a)) + φ, but due to the relationship that the compensation circuit has the inverse characteristic of the power characteristic, the amplitude is a and the phase difference is 0, that is, the same as the input signal. Returning to the point, the nonlinear distortion becomes smaller.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the digital modulator of the present invention is shown in FIGS. In FIG. 1, reference numeral 1 is a mapping circuit having a function of inputting a digital signal and outputting an in-phase signal I and a quadrature signal Q corresponding to the digital signal, and a reference numeral 2 is an inverse characteristic of the phase characteristic and the amplitude characteristic of the power amplifier 8. The compensating circuit has a code 3, a waveform shaping circuit having a function of shaping the waveform of the input signal so that intersymbol interference does not occur, and a code 4 inputs the waveform-shaped in-phase signal I and quadrature signal Q to perform quadrature modulation. A quadrature modulation circuit having a function of performing, a reference numeral 5 is a frequency conversion circuit having a function of converting a signal frequency from an intermediate frequency to a high frequency for transmission, and a reference numeral 6 is a ground station power amplification circuit having a function of amplifying power of a high frequency signal. Reference numeral 7 is a frequency conversion circuit having a function of converting the frequency of a signal from the ground into a transmission frequency of a satellite, and reference numeral 8 is a power amplification circuit having a non-linear characteristic. Reference numeral A is the basic configuration of the digital modulator of the present invention. Further, in FIG. 2, reference numeral 9 is a delay circuit having a function of delaying a digital signal, and reference numeral 1
Reference numeral 0 is a compensation amount correction circuit having a function of adjusting the compensation amount of the compensation circuit 2 by the digital signal from the delay circuit 9. Reference numeral A'denotes a configuration of a digital modulator including the delay circuit 9 and the correction circuit 10. The operation of the digital modulator of the present invention will be described below with reference to FIGS. 1 and 2.

The compensating circuit 2 having the inverse characteristic of the phase characteristic and the amplitude characteristic of the power amplifier 8 is connected before the waveform shaping circuit 3. The digital signal is input to the mapping circuit 1, and its output is input to the compensation circuit 2. The output of the compensation circuit 2 is
A signal having a wide bandwidth converted by the inverse characteristic of the power amplifier 8 is output. The waveform of this signal is shaped by the waveform shaping circuit 3, the bandwidth is limited, and the signal is output. This signal is sent from the ground station to the satellite and amplified by the power amplifier 8 mounted on the satellite. At this time, the signal is distorted due to the non-linearity of the power amplifier 8, but the distortion is small because the signal is previously compensated by the inverse characteristic.

A modulation system is set to 16 QAM, the characteristics of the power amplifier shown in FIG. 4 are used, and the result of a computer simulation in an equivalent low frequency system shows that the operating point of the power amplifier is changed without using a compensation circuit. It was improved by 3.7 dB as compared with the case where it was left alone. In this case, twice as much information can be transmitted in the same frequency bandwidth as in the conventional QPSK modulation.

Further, the compensation circuit 2 may be connected to a compensation amount correction circuit 10 as shown in FIG. The correction circuit 10 adjusts the amount of compensation by the output of the delay circuit 9. The operation of the correction circuit 10 will be described below.

In the operation of the compensating circuit 2 described above, since the power amplifier 8 is non-linear, the compensating circuit 2 having the opposite characteristic is also non-linear. Therefore, intersymbol interference of digital signals occurs, which has not occurred in the linear circuit. That is, the non-linear influence differs depending on the arrangement of the digital signals. Therefore, by delaying the input digital signal by the delay circuit 9, a signal of several words before and after the modulated signal is taken out, and the intersymbol interference occurring in the non-linear circuit depending on the arrangement of the signals is known in advance. The deviation of the amplitude and the phase due to the interference can be adjusted by the compensation amount correction circuit 10.

The output of the delay circuit is D1, D2, D3, ...
-Dn, amplitude deviation due to intersymbol interference is α (D1, D
2, D3, ... Dn) and the phase shift due to intersymbol interference is ψ (D1, D2, D3, ... Dn),
Correct the amplitude characteristic C (a) of the compensation circuit to C (a) -α,
The phase characteristic φ (a) is corrected to φ (a) −φ. The modulation system is 16QAM, the one shown in FIG. 4 is used as the characteristic of the power amplifier, four delay circuits 9 are used, and the result of the computer simulation in the equivalent low-frequency system shows that the correction circuit is not used. It was improved by 1.6 dB.

As described above, when the digital modulator of the present invention is used, it can be transmitted with a small distortion even in satellite broadcasting and satellite communication using a non-linear power amplifier, and QAM and AP can be transmitted.
Modulation such as SK (amplitude phase modulation) in which information is added to the amplitude is also possible.

The compensation amount correction circuit can be realized by sequentially calculating the amount to be corrected by the digital signal processor. Alternatively, the amount to be corrected may be calculated in advance and the data may be recorded in an analog element or a read-only memory.

[0018]

Since the present invention is configured as described above, it has the following effects.

In a line using a non-linear power amplifier such as satellite broadcasting and satellite communication, the non-linear compensation of the signal is performed without changing the operating point of the power amplifier and without widening the bandwidth of the transmission signal. Reduce the effect of interference between
Digital signals can be efficiently transmitted.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing an embodiment of the present invention.

FIG. 2 is a circuit configuration diagram showing an embodiment in which a compensation amount correction circuit is added.

FIG. 3 is a circuit configuration diagram of a transmission system having a conventional compensation circuit.

FIG. 4A is a diagram of an amplitude characteristic example of a power amplifier, and FIG. 4B is a diagram of a phase characteristic example of a power amplifier.

5A is a diagram of an amplitude characteristic example of a compensation circuit, and FIG. 5B is a diagram of a phase characteristic example of a compensation circuit.

FIG. 6 is a code layout diagram showing the operation of the compensation circuit.

[Explanation of symbols]

1, 11 mapping circuit 2 compensation circuit 3, 12 waveform shaping circuit 4, 13 quadrature modulation circuit 5, 14 frequency conversion circuit 6, 15 ground station power amplification circuit 7 frequency conversion circuit 8 power amplification circuit 9 delay circuit 10 compensation amount correction Circuit 20 Signal point before non-linear effect 21 Signal point affected by non-linearity 22 Signal point compensated by compensation circuit 100 Conventional compensation circuit A Basic configuration of digital modulator A'Including delay circuit and correction circuit Configuration of digital modulator I In-phase signal Q Quadrature signal

Claims (5)

[Claims] 1. A mapping circuit for inputting a digital signal and outputting an in-phase signal and a quadrature signal, a compensating circuit having inverse characteristics of the amplitude characteristic and the phase characteristic of a power amplifier used for power amplification of a transmission signal to be received. And a waveform shaping circuit, and a quadrature modulation circuit having an output of the waveform shaping circuit as an input. 2. A plurality of delay circuits for inputting and delaying a digital signal, and a correction circuit for correcting a compensation amount by an output signal of the delay circuit are added, and an output of the delay circuit is used as an input of the mapping circuit. The digital modulator according to claim 1. 3. The digital modulator according to claim 2, wherein the compensation circuit and the compensation amount correction circuit are configured by analog elements. 4. The digital modulator according to claim 2, wherein the compensation circuit and the compensation amount correction circuit are constituted by a digital signal processor. 5. The digital modulator according to claim 2, wherein the compensation circuit and the compensation amount correction circuit are constituted by a read-only memory.